Fiber Bragg gratings (FBGs) are optical filters that are inscribed into the core region of a single mode optical fiber waveguide using high powered laser systems. Bragg gratings have found many applications in the telecommunication industry as optical filters for dense wavelength division multiplexing application because of their compatibility with other optical network components. They have been used as add/drop multiplexers, applied for stabilization of pump lasers used in Erbium doped optical amplifiers, flattening the spectral gain responses of optical amplifiers and for compensation of chromatic dispersion in legacy optical fiber networks.
Aside from their extensive applications in the telecom domain, FBGs have also demonstrated themselves to be attractive devices for sensing temperature and strain along an optical fiber. Variations in the spectral response of the grating result from period changes in the Bragg grating due to strains or temperature variations that are experienced by the in-situ optical fiber. These FBG sensors offer important advantages over other sensor technologies because of their electrically passive operation, electromagnetic interference (EMI) immunity, high sensitivity and multiplexing capabilities. Fiber Bragg gratings are simple, intrinsic sensing elements which traditionally have been UV photo-inscribed into photosensitive Ge-doped silica fiber. Each FBG sensor has a characteristic retro-reflective Bragg resonance or Bragg wavelength, which is dependent upon the periodicity of the grating photo-inscribed within the fiber and the effective refractive index neff, of the fundamental core mode of the optical fiber. The FBG sensors can then easily be multiplexed in a serial fashion along a length of single fiber. When embedded into composite materials, optical fibers with an array of FBG sensors allow for distributed measurements of load, strain, temperature and vibration of the material creating what has is commonly referred to as “smart structures” where the health and integrity of the structure is monitored on a real-time basis.
Typically fiber Bragg gratings are fabricated using high powered UV-laser sources and a multi-step process which include:                1) Photosensitization of Ge-doped fiber by ‘hydrogen-loading’ taught by Atkins et al. in U.S. Pat. No. 5,287,427,        2) Cooling of the optical fiber to prevent de-photosensitization due to outgassing of hydrogen, for fiber storage,        3) Mounting the fiber into a writing system,        4) Connecting the fiber to an optical measurement system,        5) Removal of the UV-laser-absorbing protective polymer coatings of the optical fiber,        6) Inscription of the FBG by exposing the UV-photosensitive core of a germanium doped silica core optical fiber to a spatially modulated UV laser beam in order to create permanent refractive index changes in the fiber core. Such a spatially modulated UV beam can be created by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al. The techniques taught by Glenn and Hill result in gratings that are typically referred to as Type I gratings.        7) Collection and logging of data        8) Recoating of the stripped region of the optical fiber        9) Removal of the fiber from the writing system        10) Heating of the fiber and grating to outgas remaining hydrogen and stabilize the grating response by removing thermally unstable laser induced index change in the waveguide.        
Several examples of prior art methods of automating some of these process steps are available. Novack et al. in U.S. Pat. No. 6,272,886 describes an automated optical fiber spool reel-to-reel FBG inscription system where a fiber under tension is translated through various stations that performing some of the multi-step processes listed above. Specifically a spooling apparatus under tension control pays out fiber, passes the fiber through a fiber stripping chamber; once stripped the fiber continues to a writing head where portions of the fiber still possessing a coating are clamped in front of a phase mask The fiber is then exposed to UV radiation from an excimer laser. Additional tension can be applied locally between these clamps in order to do limited tuning of the grating. After inscription the write head clamps release and the fiber is then translated to an annealing chamber. After annealing, the fiber is then translated to a fiber recoating station and coating cure chamber before it is received by the take-up spool. A schematic figure of the inscription system taught by the inventors of U.S. Pat. No. 6,272,886 is presented in FIG. 1 of this applications as an example of prior art.
Burt et al. in U.S. Pat. No. 6,522,808 describes a system for multiple writing stations whereby a beam from a single UV laser source is subdivided into separate beams and redirected to separate writing stations with jigging available to mount preprocessed optical fiber (stripped of its polymer coatings and hydrogen loaded). Beams can be manipulated to perform grating inscriptions separately. Afterwards, post processing of the fiber (recoating, annealing) are performed elsewhere. A serious limitation of this patent is that the time consuming and potentially degrading processes of fiber stripping and recoating are not solved.
Automated UV grating inscription systems require complex fiber handling because of the necessity to hydrogen loading, stripping/recoat fibers, post process annealing. U.S. Pat. Nos. 6,487,939, 6,503,327, and 6,532,327 assigned to 3M Innovations teach methods to strip and handle fibers for UV laser exposure by mounting the fibers in special cassettes; expose the cassette mounted fiber to a UV laser in order to inscribe a Bragg grating and then how the stripped fiber containing the grating can be recoated while remaining in the fiber management cassette.
Alternatively another automated FBG production line is taught by Lefebvre in a series of U.S. Pat. Nos. 6,778,741, 6,934,459 and 7,164,841 where a reel of optical fiber is unwound and mounted in a fiber support jig described in U.S. Pat. No. 6,778,741. The optical fiber photosensitized with hydrogen as taught by Atkins et al. in U.S. Pat. No. 5,287,427 is then transported to processing stations for the fibers stripping, exposure to the UV laser, and recoating.
A critical element of an automated FBG process relies on the precise alignment of the spatially modulated UV laser beam onto the fiber core. Komukai et al. in IEEE Photonics Technology Letters 8 (11) p. 1495 (1996) describe a method of inducing photoluminescence from GeO defects in a germanium doped optical fiber core using UV radiation. The amount of UV-generated 400 nm photoluminescence is proportional to amount of the spatially modulated UV laser beam that overlaps with the core of the Ge-doped optical fiber. Since the photoluminescence in the blue occurs at a longer wavelength (lower energy) than the UV absorption wavelength, it is often referred to as fluorescence. This photoluminescence can be guided by the core. By monitoring the level of this guided photoluminescence at the end face of the optical fiber the overlap of the UV beam with the fiber core can be determined. This UV beam/fiber core overlap can then be optimized through a feedback loop. By monitoring the UV induced photoluminescence in the fiber core through the fiber end face, Nishiki et al. in U.S. Pat. No. 5,914,207 teach a method of probing different sections of length of fiber with a UV beam in order to optimize fiber tilt. Lefevbre in U.S. Pat. No. 6,778,741 teaches a variation of this technique whereby the visible photoluminescence that is emitted radially from the irradiated core of the optical fiber, instead of that guided along the fiber core, is monitored by a detector placed adjacent to the exposure region of the fiber.
To bypass the necessity of stripping the optical fiber before UV grating inscription, Askins et al in U.S. Pat. No. 5,400,422 teach a method of inscribing gratings while the fiber is being pulled on the draw tower but before the fiber is coated. Using a holographic exposure set up and a single high energy UV pulse, a high reflectivity, high thermal stability damage grating or Type II grating can be written on the fly. A serious disadvantage of this approach is the necessity to have a fiber draw tower in order to manufacture an optical fiber grating or fiber grating array.
A limitation of these prior-art automation systems for FBG manufacturing based on UV lasers is that they rely on several processing steps that are time consuming and can potentially degrade the integrity of the optical fiber reducing the yield of the manufactured FBGs. They include the necessity to use Ge-doped optical fiber, to hydrogen load to increase fiber photosensitivity, to strip the fibers of their protective polymer coatings and then reapply them after FBG inscription. Mihailov et al. in U.S. Pat. Nos. 6,993,221 and 7,031,571 (both of which are herein incorporated by reference) teach methods of fabrication of FBGs using high power ultrafast pulse duration radiation and a phase mask. By using infrared femtosecond duration laser pulses, they teach that gratings can be written in the cores of non-UV photosensitive as well as photosensitive fibers directly through protective polymer coatings and without the necessity for hydrogen loading. It is not limited to specialty optical fibers but optical fibers that are readily available commercially. The mechanism by which index change is induced in the fiber is not one of linear absorption of high energy UV photons but nonlinear simultaneous absorption of lower energy infrared photons.
In the present application, references to “a permanent change in an index of refraction within a core of the optical waveguide” represents the formation of a grating that is stable at higher temperatures, for example at a temperature up to just below the glass transition temperature of the material forming the optical waveguide being inscribed with the grating. This is also referred to herein and in the art as a type II grating. In one embodiment, where the waveguide is a silica based fiber, a permanent change in an index of refraction within a core of the optical waveguide is one which is stable at temperatures of up to 1000° C. In other embodiments, where the optical waveguide comprises a different material (e.g. crystalline sapphire), the temperature may be higher than 1000° C.
In the present application, references to a “non-permanent grating” refer to gratings that are not stable to high temperatures, and that can be substantially removed by heating the optical waveguide. These are also referred to herein and in this field of art as type I gratings. In one embodiment, a non-permanent grating is one which is substantially removed by heating at temperatures of up to 1000° C. In some embodiments, the non-permanent gratings are substantially erased at temperatures lower than 1000° C., for example 800 C
It is an object of this disclosure to overcome the aforementioned limitations within the prior art systems for automated fabrication of fiber Bragg gratings by presenting methods and a system for automated FBG inscription that utilizes but is not limit to femtosecond pulse duration infrared lasers.